Assuming argonians are closely related to earth's amphibians, they could have a lifespan somewhere between 15 to 100 years. Small amphibians generally live at least 10 or 15 years, while large amphibians like giant turtles can live up to and sometimes over 100 years.
here are some articles on lifespans of Amphibians:
http://www.teachervision.fen.com/reptiles/amphibians/8690.html
http://www.newton.dep.anl.gov/natbltn/400-499/nb486.htm
http://www.wonderquest.com/LifeSpan-MaxMin.htm
First off I'm no expert. That being said based on the Oblivion game, other games with reptial races and real animals I would give Argonians a life span of 200-250 years. Turtles for example live 200+ years, Crocs 300+ years. So 200-250 years makes sense to me.
Turtles very rarely ever live past 150ish and crocodiles live about 70-80 usually, though it has been reported for one to live 115 years. Check the links above.
Also, the oldest animal ever was a 400 year old clam if anyone was wondering.